This invention relates to an infrared ray-absorbing quartz glass which can be used under high temperatures, and also to a process for making the same. The glass of this invention is well adapted to the use, for example, as bulb glass.
Conventional infrared ray-absorbing glasses include phosphate glass and ordinary silicate glass containing Fe.sup.++.
Those known glasses, however, are defective in the following points.
The infrared-absorbing glasses produced by incorporating iron into phosphate glass, soda-lime glass, borate glass, lead glass, aluminosilicate glass and the like, invariably exhibit poor heat resistance and chemical stability. Particularly when they are used under high temperatures over a prolonged period, their light transmittance is impaired. Furthermore, due to their poor heat resistance, they cannot be used in the vicinity of heat-generating sources such as a light source. Consequently, the devices using said glasses must have large sizes, and hence the infrared-absorbing glasses employed in such devices must serve as large size filters.
In the process for making an infrared-absorbing glass by impregnating a high silica, porous glass obtained from borosilicate glass, with an iron salt and sintering the same, the produced glass inavoidably contains boron which drastically reduces the heat resistance of quartz glass. Thus the product would withstand the temperatures of, at the most, around 800.degree. C., exhibiting heat resistance below that of quartz glass. Also the process must be skillfully and precisely controlled for satisfactory dehydration of the iron salt penetrating into the fine voids in the porous vitreous body and decomposition of said salt into oxide. If the sintering is effected before the impregnated system is sufficiently dried, voids are apt to be formed or the vitreous body broken, rendering the process quite impractical.